[clang] [NFC][analyzer][docs] Migrate 'annotations.html' to RST (PR #122246)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

> Shouldn't we leave a stub in the old HTML page that the docs were moved? Or we haven't done that in the past either?

This commit _does_ leave a stub HTML page (which is closely modeled after the earlier similar stub pages).

> I understand (and strongly agree with) you not making any meaningful changes to the docs page you moved. With that said, later down the line, we should make sure that checkers are called "checkers", and not "checks" (the latter is used in clang-tidy), and that `ownership_.*` attributes must be mentioned here as well.

OK, I'll create a follow-up commit for these. Where should we _primarily_ document the `ownership_.*` attributes: here or in the global documentation of attributes? (We must avoid duplicating the same content twice, so one location should have the full documentation (which you've written recently) while the other should contain a very brief description (1-2 sentences) and a link to the full docs.

> Otherwise LGTM, thanks for the work!

I'm merging this commit now; if anybody else has any suggestions, we can discuss them in follow-up PRs.

https://github.com/llvm/llvm-project/pull/122246